To measure transparent or partially transparent samples, for example of the human eye, short-coherence interferometers which operate by means of optical coherence tomography (hereinafter: OCT) are known, for example from WO 2007/065670 A1. They serve to detect the location and size of scattering centres within a sample, such as for example miniaturized optical components or biological tissue, e.g. the human eye. For an overview of the corresponding literature on OCT, reference may be made to US 2006/0109477 A1. This patent publication also describes the basic principles of OCT.
The principle of OCT comprises both embodiments in which an imaging occurs through irradiation and radiation detection by scanning at different locations across the direction of incidence of the radiation and embodiments, simplified compared with these, in which the irradiation and radiation detection is carried out only along an axis that remains unchanged and axial (i.e. 1-dimensional) scattering profiles are thus generated. The latter embodiment corresponds, as far as the image production is concerned, to a so-called A-scan of ultrasound image production; it is also called optical coherence domain reflectometry or short-coherence reflectrometry (OCDR). When OCT or OCDR is mentioned here, they are to be understood to mean both scanning and OCDR systems.
To detect larger objects or to enlarge the measurement range, it is known to use an interference of several measurement beams each with a separate reference beam or to superimpose several individual measurement beams in pairs, which is also called a so-called “dual beam” interferometer. This is known from Drexler et al., “Submicrometer Precision Biometry of the Anterior Segment of the human Eye”, Investigative Ophthalmology & Visual Science, June 1997, Vol. 38, No. 7.
Time-domain OCDR (TD-OCDR) with rapid-scanning reference arm and Fourier-domain OCDR (FD-OCDR) with fixed reference arm and evaluation of spectral interferences are known as variants for OCDR. The latter exists in yet another variant using broadband light sources and spectrometer-based detection (spectral domain or SD-OCDR) and in another variant using spectrally tunable light sources and broadband detectors (swept-source or SS-OCDR).
In TD-OCDR, the sample is illuminated by a short-coherent radiation and an interferometer ensures that radiation scattered back from the sample can interfere with radiation which has passed through a reference beam path. This principle, already described at a relatively early stage in Huang, et al., Micron-Resolution Ranging of Cornea Anterior Chamber by Optical Reflectometry, Lasers in Surgery and Medicine 11, 419-425 (1991), can record a depth-resolved scattering profile of the sample if the length of the reference beam path is adjusted, whereby a window corresponding to the coherence length of the radiation used is adjusted in the sample. If, as described in Huang, et al., Science 254: 1178-1181, 1991, the illuminating beam is also moved relative to the sample, a depth-resolved sectional view (tomogram) of the sample can be produced, i.e. lateral scanning optical coherence tomography (TD-OCT) is realized. The size of this window defines the maximum achievable depth resolution. For a good depth resolution, radiation sources with the shortest possible coherence, i.e. spectrally wide, are thus necessary. Because of the measurement method, only a fraction of the radiation reflected back, i.e. that scattered back from the measurement depth of the sample, which corresponds to the length of the reference beam path, is detected at any time. In known structures, therefore, over 99% of the photons scattered back from the sample are not actually detected for the measurement.
A higher yield is obtained with SD-OCDR. Here, the length of the reference beam path is no longer altered; instead, the radiation brought to interference is detected spectrally resolved. The depth information of the sample, i.e. the depth-resolved contrast signal, is calculated from the spectrally resolved signal. As a mechanism for adjusting the path length of the reference beam path is no longer necessary, the SD-OCDR technique is capable of measurement simultaneously at all depths of the sample. The thereby achieved higher yield of the radiation scattered back achieves a sensitivity up to 20 dB higher for the same measurement time. A disadvantage of SD-OCDR systems is the maximum measurement range size, which is limited by the spectrometer resolution, and the reduction in sensitivity which increases with the measurement depth. The required structure is also much more expensive.
The SS-OCDR variant, in which the spectral resolution of the interference signal with a spectrometer is dispensed with and, instead, the illumination source is spectrally tuned, requires somewhat less additional structural outlay. This method, like SD systems also, is more sensitive than TD-OCDR because of the higher photon recovery, as M. Choma et al. explain in “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express 11, 2183-2189 (2003). In the case of SS-OCDR, the maximum resolution corresponds to the tunable wavelength range of the radiation source, and the measurement depth, i.e. the axial measurement range, is predetermined by the coherence length, i.e. the line width, of the radiation used.
A particularly compact SS-OCDR variant is to be found in S. Vergnole et al., “Common Path swept-source OCT interferometer with artifact removal”, in Coherence Domain Optical Methods and Optical Coherence Tomography in Biomedicine XII, ed. J. Izatt et al., Proc. Of SPIE Vol. 6847, 68472W, 2008. Here reference radiation is produced by reflection at the end of a partly-reflective light-conducting fibre which conducts the measurement radiation onto the sample. The radiation reflected back from the sample and the reference radiation are then conducted through an interferometer the arms of which have different path lengths, and then superimposed. The path length difference defines the distance of the covered sample region from the partly-reflective end of the light-conducting fibre. To detect distances between different sample regions beyond the OCDR measurement depth, several measurements must be carried out. The use of a fibre-tip reflector as reference is also known from US 2007/0008545 A1.
In all OCDR variants, the measurement range and the measurement resolution and/or measurement accuracy are linked in a certain way. For example, in SD-OCDR the ratio between realizable spectrometer resolution and spectral width to be covered is limited. In TD-OCDR, although an adequate spectral bandwidth or sufficiently short coherence length of the radiation must also allow the resolution of sample structures of interest, the measurement range is ultimately linked via the maximum realizable scanning speed and sampling frequency to the measurement sensitivity and measurement resolution for a given measurement duration. To rectify the limitation imposed by the link, WO 2007/065670 A1 skilfully describes combining several interferometer devices which are each assembled from their own reference beam path as well as an associated sample beam path. By different matching of these several interferometer devices which, although combined in one device, are independent, measurements can be taken simultaneously at different points in the eye and thus the measurement range can be enlarged. The document further describes different approaches for differentiating the radiations in the combined interferometers, for example in respect of the polarization of the radiation or the wavelength. Such a differentiation is also described in WO 2001/038820 A1 which, however, is concerned only with TD-OCDR, thus requires moving elements for adjusting the length of the reference beam path. The principle of using several reference beam paths of different lengths can also be found in US 2005/0140981, or in U.S. Pat. No. 6,198,540, which each relate to TD-OCDR for enlarging the measurement range and use several, individually adapted reference beam paths of different lengths.
The outlay increases dramatically when a sample subjected to movements or pulsations, such as e.g. the human eye, is to be measured. Measurement must then either be carried out so rapidly that no disruptive movements or pulsations occur in the measurement duration, or such influences must be compensated.
One possibility for compensation is the already mentioned dual-beam structures/methods. Here, a measurement beam is constructed from two beam portions which would be capable of interfering with each other if they were not axially shifted towards each other. This axial shifting of the portions of the incident measurement radiation is generated by a tuning interferometer and adjusted to fit the axial distances between the sample regions to be measured. The backscatter or reflection back on the sample then removes the axial offset for the radiation portions again and the sample radiation can then interfere directly. An interference signal is thus obtained according to the known OCDR principles.
The already named US 2006/0109477 ultimately does not actually allow several regions of a sample to be covered which are axially further apart than the measurement depth, but concentrates instead on achieving the greatest possible sensitivity, for which 2×2 and 3×3 fibre couplers combined with a difference signal evaluation of quadrature components are used to realize the balanced detection.